1. Field of the Invention
This invention relates to a decoding apparatus and method using an LDPC (Low Density Parity Check) code employed in, for example, a communications system, information input/output system and the like.
2. Description of the Related Art
As is well known, an LDPC code is a block code defined by a sparse check matrix and is decoded by repetitive decoding. The LDPC code is created on a bipartite graph, and has a characteristic that a structure of the bipartite graph gives a great influence to a transmission error rate characteristic.
It is known that the LDPC code can form an efficient encoder by optimizing the structure of the bipartite graph. A method of forming a substantially optimum encoder is now introduced into a binary symmetric communication channel (transmission path between the transmitter and the receiver).
In addition, a method of obtaining an optimum encoder structure by executing feedback from a receiving side to a transmitting side, in a channel other than the binary symmetric channel or a channel changed in time or frequency, is conceived (for example, Jpn. PAT. Appln. KOKAI Publication No. 2003-198383).
Examples of the channel changed in time are CDMA (Code Division Multiple Access), OFDM (Orthogonal Frequency Division Multiplexing), and the like as employed in the mobile communications including fading and the indoor communications including many multi-paths.
Examples of the modulating method of the channel other than the binary symmetric channel are M-value PSK (Phase Shift Keying), M-value QAM (Quadrature Amplitude Modulation), M-value ASK (Amplitude Shift Keying), M-value AMPM (Amplitude Modulation-Phase Modulation), M-value PPM (Pulse Position Modulation), and the like.
In the above-explained method of executing feedback from the receiving side to the transmitting side, however, both the transmitting (encoding) side and the receiving (decoding) side need to reconstruct the system in accordance with the conditions of the channel, and a feedback channel allowing the conditions of the channel to be transmitted from the receiving side to the transmitting side needs to be secured. For this reason, if the encoder structure is optimized in the conventional method, the efficiency of use of the channel is low and the speed of adapting to the conditions of the channel is slow.